The present invention relates to a synchronizing signal detection circuit which detects a corresponding synchronizing signal from a video signal including synchronizing signals.
A patent document 1 (Japanese Unexamined Patent Publication No. 2001-275014) has disclosed a video signal processing device which, even though a pedestal level of an analog video signal inputted thereto varies, outputs a digital video signal whose pedestal level is always controlled to a preset black level. Using such a device makes it possible to avoid abnormalities in the hue of a reproduced image.
Meanwhile, a synchronization failure or error such as video blurring caused by an inability to accurately detect a synchronizing signal contained in a video signal due to the influence of high-frequency noise or the like becomes a problem as well as a problem associated with an abnormality in brightness such as a hue abnormality that causes an abnormality in video reproduction.
FIG. 1 shows a synchronizing signal detection circuit for a conventional video signal. In the figure, an input video signal S10 inputted from a video signal input terminal 10 is inputted to an IIR filter unit 11. The IIR filter unit 11 outputs a video signal S11 obtained by eliminating high-frequency noise components from the input video signal S10. The video signal S11 is inputted to a synchronizing signal extraction unit 21, which detects a synchronizing signal contained in the video signal S11 and outputs it as a synchronizing signal S21.
However, error detection of a synchronizing signal still occurs depending on such related arts. Since the difference in level between effective data and a synchronizing signal is large where, for example, the end of an effective data period of a video signal is terminated at high-brightness data, the synchronizing signal is rounded in its waveform and erroneously detected depending on the effective data when it is caused to pass through an IIR filter. The error detection of the synchronizing signal leads to synchronization errors and becomes the cause of occurrence of video blurring.